Cast polymerization methods are known methods of molding plastic lenses. For example, Megane (published May 22, 1986 by Medical Aoi) discloses a manufacturing process for diethylene glycol bisallyl carbonate lenses (CR-39 lenses). This lens manufacturing process describes a casting mold in which a gap is maintained between the upper mold and lower mold of a glass master mold by means of a cylindrical gasket to form a cavity. A lens starting material liquid is cast into this cavity. Following casting, the mold is placed in an electric furnace and heated to conduct polymerization. The fully polymerized lens is then removed from the mold.
Example of methods of casting a starting material liquid into a cavity is a method in which a gasket provided with a casting inlet is employed and a starting material liquid is cast through the inlet. For example, Japanese Utility Model Publication (JIKKO) Heisei No. 6-39951 (referred to as “Reference 1”, hereinafter) describes such a gasket. In the casting of the starting material liquid into the casting mold employing this gasket, the starting material liquid is cast under its own weight by a flowing action from the casting inlet part in an arrangement where the gasket is tilted so that the casting inlet of the casting inlet part faces upward. When casting a starting material liquid from above as is the case with this gasket, bubbles are sometimes involved during casting, depending on the casting conditions and materials. When the viscosity of the starting material liquid is low, there are few problems because the bubbles are relatively easily removed. However, when employing a high viscosity starting material liquid or a starting material liquid with a high initial polymerization rate, the removal of bubbles is difficult. A gasket of such configuration cannot be utilized when employing such starting materials.
In molding methods employing the gasket disclosed in Reference 1, the gasket is removed following polymerization from a casting mold that has been filled with starting material. In that process, it is necessary to sever, in the vicinity of the casting hole, the portion that has polymerized within the starting material liquid casting inlet and remove it from the molded article. When employing a gasket such as that described in Reference 1, the portion that has polymerized in the vicinity of the casting hole is usually broken by bending the casting inlet, and the gasket is removed. However, when manufacturing a high-strength molded article, it is difficult to break the portion that has polymerized in the vicinity of the casting hole after the polymerization is finished. Further, if employing a starting material liquid with high viscosity, when the casting hole is widened to facilitate casting, the strength of this portion becomes even greater and breaking becomes even more difficult. Thus, there is a need for a molding method permitting the easy removal of the gasket.
As an example of a method of manufacturing plastic lenses with a casting molding from a highly viscous lens starting material liquid with a rapid initial rate of polymerization, International Patent Publication WO 03/084728 (referred to as “Reference 2”, hereinafter) discloses a method of molding plastic lenses by casting polymerization employing an isocyanate terminal prepolymer component (A) in the form of the reaction product of an aliphatic diisocyanate having an intramolecular cyclic structure and a diol having an average molecular weight of 300 to 2,500, and an aromatic diamine component (B) that react rapidly when mixed. The molded article obtained by molding based on the method disclosed in Reference 2 is of great strength, rivaling that of polycarbonate. In the lens manufacturing process described in Reference 2, a plastic lens is manufactured by casting a starting material liquid into a cavity formed with an upper mold, a lower mold, and a gasket having a casting hole and a discharge hole; severing the polymerized portion inside the cavity from that outside the cavity after a prescribed period has elapsed; subsequently introducing the lens into an electric furnace where it is heat polymerized; and removing the lens from the mold once polymerization has concluded.
In the molding of lenses using components (A) and (B) described in Reference 2, these components are desirably rapidly mixed and then immediately cast into the casting mold after mixing. In particular, since component (A) is of high viscosity, it is difficult to remove any bubbles that generate in the cavity. Further, bubbles that are generated tend not to rise upward, but remain within the molded article. Further, since polymerization of components (A) and (B) commences immediately after mixing and polymerization rate is high, the removal of bubbles becomes even more difficult.
In the method described in Reference 2, the starting material liquid is cast using a nozzle equipped with a casting inlet sealed part in an inclined state with the discharge outlet of the gasket on top. As this occurs, the starting material liquid fills the cavity from bottom to top. Casting from the bottom in this manner effectively reduces the quantity of bubbles entrained relative to cast from the top.